Bimolecular Deactivation Processes

In addition to monomulecular processes such as emission and radiationless deactivation there are very important bimolecular deactivation mechanisms 

Fluorescence quenching is a very general phenomenon that occurs through a variety of different mechanisms. All chemical reactions involving molecules in excited states can be viewed as luminescence quenching. Such photochemical reactions will be dealt with in later chapters. 

Photophysical quenching processes that do not lead to new chemical species can in general be represented as 

Most intermolecular deactivation processes are based on collisions between an excited molecule M and a quencher Q. They are subject to the Wigner-Witmer spin-conservation rule according to which the total spin must not change during a reaction (Wigner and Witmer, 1928). 

In order that the products C and D lie on the same potential energy surface as the reactants A and B. the total spin has to be conserved during the reaction. The spins and of the reactants may be coupled according to vector addition rules in such a way that the total spin of the transition state can have the following values  

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This equation shows that the reaction rate is neither first-order nor second-order with respect to species A. However, there are two limiting cases. At high pressures where [A] is lar e, the bimolecular deactivation process is much more rapid than the unimolecular decomposition (i.e., /c2[A][A ] /c3[A ]). Under these conditions the second term in the denominator of equation 4.3.20 may be neglected to yield a first-order rate expression. 

A very important bimolecular deactivation process is the electronic energy transfer (ET). In this process, a molecule initially excited by absorption of radiation, transfers its excitation energy by nonradiative mechanism to another molecule which is transparent to this particular wavelength. The second molecule, thus excited, can undergo various photophysical and photochemical processes according to its own characteristics. 

The almost linear relationship between Iq/I and oxygen pressure indicates that, when the fluorophore is entangled or trapped in silicone, the only bimolecular deactivation process is oxygen quenching. From the temperature dependence of quenching, the solution enthalpy 8H was calculated to be —3.0 kcal mol. ... 

Although catalysts supported by the above approach usually behave similarly to those from homogeneous systems and yield polymers with essentially the same properties, the equivalents of MAO vs the catalyst precursor can be reduced significantly—down to 100—500 equiv, compared to 10 —10 equiv in a homogeneous systems. This behavior has been rationalized with the hypothesis that because the silica surface is essentially coated with MAO molecules, the weak ion pairs may be able to float over the surface much like in solution, thus resulting in a similarity between this type of system and the catalyst in solution. The difference in MAO equivalents required, however, may be attributed to the fact that immobilization of the zirconocenium species may partially or completely inhibit bimolecular deactivation processes. " The supported MAO activator can also be prepared by in situ hydrolysis of AlMes with hydrated silicas (10—50 wt % absorbed water). 

As a lagniappe, the ratio of alumoxane to metal can be reduced considerably. It has been suggested that large excesses of MAO (aluminum-to-metal ratios of 1000—10 000) are needed in homogeneous polymerizations with metallocene catalysts in order to prevent bimolecular deactivation processes (Scheme 1). 

Fig. 1 Unimolecular and bimolecular deactivation processes of an excited state.

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